The field of the present invention is semiconductor lasers, and more particuarly, large scale laser integration devices and fabrication methods therefor, including methods for fabricating optical deflection elements directly on a wafer substrate. Still more particularly, the invention relates to the fabrication of an optical input/output port in a semi-conductor laser device.
Recent attention has been directed to full-wafer technology for large scale laser fabrication and integration that have many of the characteristics of large scale integrated circuits. See, P. Vettiger et al., Full-Wafer Technology--A New Approach to Large-Scale Laser Fabrication and Integration, IEEE Journal of Quantum Electronics, Vol. 27, No. 6 (June 1991). Wafer scale laser fabrication represents a dramatic departure from the usual semiconductor laser production approach wherein significant laser processing is performed following the scribing and cleaving of the wafer substrate to form a plurality of laser mirrors. In this prior approach, important processing steps, such as mirror coating and laser testing, must be performed at the bar/chip level. It will be appreciated that the individual handling and processing of components at this level is a significant production obstacle. Large scale laser integration promises to yield considerably improved efficiencies. Among the recognized advantages of full-wafer laser fabrication are reduced handling of laser chips, automated full-wafer processing, burn-in and testing, potentially higher yields, a wide variety of new device configurations and the ability to provide on-wafer integration of auxiliary electrical and optical components.
An important requirement of large scale laser integration is the ability to fabricate mirror surfaces without cleaving the substrate. Thus, in proposed large scale laser devices, the cleaved mirror facet is replaced by an etched groove whose faces form the mirror facets. A conventional etched construction is shown generally in FIG. 1, which illustrates a semiconductor laser 2 whose construction, by way of example, may be that of an AlGaAs single-quantum well ridge laser. The laser 2 may be constructed from a wafer substrate 4 formed from a plurality of semiconductor p and n layers grown epitaxially. Positioned between the p and n layers is an active quantum well layer 6 disposed within a ridge waveguide structure 8. Positive and negative conductive elements 10 and 12 are respectively positioned on the upper and lower surfaces of the substrate 4 to provide forward biasing. The quantum well layer 6 confines the minority carriers in a direction perpendicular to the junction formed with the p and n layers, and modifies the density of states such that very efficient lasing action is realized. Optical confinement parallel to the junction plane is provided by the ridge wave guide. When the laser 2 is energized, the laser beam 14 emerges from the laser mirror facets 16 formed by the etched grooves 18. Either of the opposing facets 20 of the grooves 18 may be formed as part of an integrated monitor diode.
In conventional wafer scale fabrication, the wafer substrate 2 is formed with a plurality of parallel ridge waveguide structures. A series of individual lasers are then provided by forming a plurality of etched grooves extending perpendicularly to the ridge wave guide structures. FIG. 1 illustrates one such ridge wave guide structure bounded by two perpendicularly oriented etched grooves. Typically, an integrated monitor diode will be formed at the opposing side of one of the bounding groove structures. The other groove will be cleaved down the middle to provide the laser output.
The etched grooves may be formed using a variety of highly anisotropic etching techniques. For example, wet-chemical etching, reactive ion etching (RIE), ion-beam etching, focused ion-beam etching (FIBE), reactive ion-beam etching (RIBE) and chemically assisted ion-beam etching (CAIBE) have all been proposed for producing laser mirror trenches of varying quality. Advantageously, regardless of which etching approach is favored, the use of microlithographic techniques generally enables the formation of laser mirror facets of practically any size, shape and orientation. Thus, a variety of wafer integrated laser structures may be formed, including short cavity lasers, groove coupled-cavity lasers, lenses, staggered arrays, monitor photodiodes and beam shaping devices.
Of particular interest with respect to the present invention are integrated laser mirror structures incorporating both a laser mirror and a beam deflector. In structures of this type, the etched groove includes one vertically oriented face and one angled face. The vertically oriented groove face serves as a laser mirror and emits the beam parallel to the substrate, while the angled groove face functions as a deflector to deflect the beam generally normal to the wafer. Etched mirror/deflector structures of this type have been fabricated using known beam etching methods. In accordance with conventional practice, the position of the mirror and deflector faces are defined by a photo resist mask and their orientation is defined by the ion beam's angle of incidence. Hence, whereas the laser-mirror is fabricated with a perpendicular ion beam, the deflector is fabricated (using a different mask) with the ion beam incident at a 45 degree angle. FIGS. 2a, 2b and 2c illustrate a sequence of processing steps utilizing this method. The sequence represents an approach wherein the deflector is fabricated prior to the laser mirror.
FIG. 2a shows a cross-section through a trench or well 22 whose back wall 24 will serve as the beam deflector. It has been fabricated utilizing the masking edge 26 of a mask having a first portion 28, and a 45 degree milling direction shown by the arrow m. An opposing wall 30 is formed simultaneously using the masking edge 32 of a second mask portion 34. FIG. 2b shows the next masking step wherein the masking-edge 36 of a mask 38 is used to define the laser mirror surface 40 shown in FIG. 2c. A protective mask 42 protects the deflector surface 24 from further etching. The ion etching beam is then applied at normal incidence to generate, after resist-removal, a mirrored-deflector geometry as shown in FIG. 2c.
The two principal problems with the fabrication method illustrated in FIGS. 2a-2c are illustrated in FIG. 3a. FIG. 3a shows a vertical cross-section through the mirror geometry of FIG. 2c. An active layer 42 in the wafer substrate generates a laser pulse 44 which is emitted as a diverging laser beam emitting from the laser mirror 40. The first flaw is that the deflector does not intercept the whole cross-section of the beam. This is because, being the back-side of the fabricated structure, its spacing from laser facet 40 is large. Thus, the trench 22 needs to be deep while an upper portion 48 of the beam 46 just passes above the wafer surface. As a result, the deflected beam 50 is only partial and asymmetrical. A second problem exists with regard to the optical quality of the deflecting mirror 24. With any substractive process, there occurs an erosion of the masking edge and the deflector surface quality deteriorates with increasing edge-depth. Since the deflector surface 24 is substantially larger than the mirror surface 40, its optical quality is correspondingly worse.
Accordingly, there is an identified need for a fabrication method for producing an optical mirror/deflector configuration that overcomes the above disadvantages.